Electric discharge tube



April 19, 1949- A. J. w. M.,VAN OVERBEEK I 2,467,711

ELECTRI C DI SCHARGE TUBE Filed April 10, 1946 I .2 Sheets-Sheet 1 April 1949- A. J. w. M. VAN OVERBEEK 2,467,711

ELECTRIC DISCHARGE TUBE Filed April '10, 1946 2 Sheets-Sheet 2 unmmzalllllllll f? E? O lllllllll I A? W 32 Patented Apr. 19, 1949 UNITED STATES PTENT OFFICE ELECTRIC DISCHARGE TUBE Application April 10, 1946, Serial No. 660,876

In the Netherlands July 2'7, 1945 6 Claims.

This invention relates to an electric discharge tube for the reception of frequency-modulated oscillations, which permits the detection of irequency-modulation but not or substantially not oftroublesome amplitude-modulation, and to a circuit arrangement comprising such a tube.

It is common to secure the aforesaid effect by means of a circuit comprising one or more amplitude limiting tubes followed by a network which is dependent with respect to frequency and com prises a tube for amplitude-modulation detection. Therefore, however, at least two tubes are necessary, even if both systems of the tubes could be incorporated in a single bulb.

Furthermore a, circuit is feasible in which by means of one or more tubes the strength of an electron current is successively acted upon by the similar voltage and by a voltage derived therefrom, of which the phase with respect to the initial voltage depends upon the momentary frequency thereof. However, this requires either two push-pull connected tubes, for the production of a low-frequency current or the frequencymodulation is only converted into amplitude modulation by means of a single tube, the latter then being detected again in the usual way. Moreover, both circuits suffer from the drawback that troublesome amplitude modulation is not suppressed.

According to the invention it is now feasible, by means of a single tube, to convert the relative phase displacements of two auxiliary signals, which are in harmony with the frequencymodulation of the incoming signal, into the corresponding loW-frequency oscillations. a tube, which comprises at least a cathode, an anode and two control electrodes and is suitable for use in a, device for demodulating a frequencymodulated signal, with simultaneous suppression of the amplitude modulation of this signal, whilst making use of a circuit in which ensue two auxiliary signals which are derived from the signal and have a phase-displacement with respect to each other which depends upon the momentary frequency deviation of the signal, an electroncurrent having a practically constant strength is successively completely transmitted or blocked by the two control-electrodes in the tube, with the result that an anode-current having a definite strength only flows at the moments when the momentary voltage of the auxiliary signals, each of which is supplied to one of the two con 'trol electrodes, deviate in a positive, negative or opposite sense from a definite value. Owing to the fact that the anode-current impulses have In such a constant value, which is independent of the value of the controlling voltages, if the latter exceed a definite minimum value, amplitudemodulation is not detected so that the tube acts as a limiter at the same time.

This is ensured by providing that the control electrodes practically not act upon the cathode current but are only capable of acting upon the distribution of the electron current among "the various electrodes in the tube. In this case the cathode current may then be adjmted to a constant value and the electron current having a constant strength can either or not be directed to an anode by the control electrodes. In this case this electron current may either be completely suppressed or transmitted by each of the control electrodes or be directed to the anode or to a collecting electrode by deflection. To ensure that the control of the second control electrode takes place as much as possible under the same conditions as that of the first control electrode,

an electron lens may be placed between the two control electrodes, by which lens the electrons are focussed in a focus near the second control electrode, even if the beam has already been given a slight deviation by the first controlelectrode. This permits to ensure that the two controls do practically not affect each other. To procure a small size of the tube and a sensitive control, the control electrodes may advantageously be designed as reflective-deflection control-electrodes.

The desired effect is also obtainable by means of a common tube having more than five or more grids by choosing the voltages of the various electrodes, notably that of the grids nearest the the cathode, in such a manner as to obtain an electron stream having a definite value which can be completely transmitted or completely suppressed by two control grids, it being necessary as a rule that, viewed from the cathode, each control grid is followed by a screen grid. The operation can be materially improved by matching the construction of the tube thereto. To such end care should be taken that the distances between each control grid and the next screen grid is smaller than the distance between each control grid and the preceding grid.

The positioning of the electrode generally involves that it will be placed at one side of the cathode. Consequently, the cathode need be coated by emitting material only at one side. As an alternative, it is feasible to coat a cathode with emitting material on both sides, one or more electrodes being provided in front of one side of the cathode, which have a function different from that of the electrodes which are provided at the other side and have the aforesaid function. The first-mentioned electrode may form a diode, for instance for automatic gain control or for a separate limiter. In some cases, however, it is also feasible to provide a triode or pentode system at this side which, as the case may be, may then act as an amplifier.

A multigrid tube will generally permit the desired effect to be obtained if, viewed from the cathode to the anode, the grids are connected in the following sequence: suction grid, first control-grid, first screen-grid, second controlgrid, second screen-grid and, as the case may be, this followed by a suppressor grid. It has turned out that in many cases the operation of the tube is improved by applying to the suppressor grid a low positive voltage of, say, 5 to 10 v, instead of connecting it to the cathode. Furthermore, a current limiting grid which is kept at zero potential or at a negative potential, may be provided between the cathode and the suction grid.

In order that the invention may be clearly understood and readily carried into effect it will now be described more fully be reference to the accompanying drawing given by way of example, in which Fig. 1 illustrates the principle of the tube according to the invention, and

Figs. 2, 3 and 4 show the operation of such a tube.

Figures 5, 6, '7 represent special forms of construction of a tube according to the invention.

In Fig. 1 the reference numeral l represents the bulb of the tube, 2 the cathode from which an electron stream 4 is obtained by means of a positive electrode 3, which current is successively controlled by the control electrodes 5 and 6. The electron stream 4 is either or not transmitted in the direction of the anode 8 according as to whether the electrode 5 has a positive or negative voltage relatively to a definite value. If the current is not transmitted it is collected by a positive electrode I or completely returned to the electrode 3. If the current is transmitted by the electrode 5, it is controlled anew by the electrode 5 by which it is likewise either or not transmitted according as to whether this electrode has a positive or negative voltage with respect to a definite value. This definite value is determined by the form of the characteristic curve indicating the anode current as a function of the voltage of the control electrode. can preferably be made zero, so that both the control electrodes and the cathode can be directly connected to earth. Often, however, it is advantageous to choose a negative bias, since in this case no damping is obtained in the presence of small alternating control-voltages, but damping of the input circuits is obtained in the presence of high alternating control voltages, which, in a measure yields a stabilisation of the control voltages. If the current is not transmitted by the electrode 6 it is either collected by an auxiliary electrode 7 or sent back to the other electrode 3. The current Ia in the characteristic curve shown in Figure 2 is a function of the controlling voltage Vb for each of the two control electrodes. The resulting anode current Is as a function of time is shown in Fig. 3 for each of the control electrodes in the case of the other control electrodes steadily transmitting the current. In regard to both control-electrodes jointly 4 one obtains the anode current impulses as a function of time according to Fig. 4. Fig. 4a represents the anode current impulses when the controlling voltages of the electrodes 5 and 6 exhibit a small phase displacement, Fig. 4b illustrating the same in regard to a phase displacement of about 90 and in Fig. 4c the phase-displacement nearly amounts to 180. It is obvious that the surface area of the figures in solid lines is proportional to the average anode current and also linearly depends upon the phase difference of the control electrodes 5 and 6. Moreover, the value of the anode-current impulses is independent of the value of the control voltages, so that the tube acts as a limiter. From the antenna in Fig. 1 the incoming signal, which is modulated in frequency, is supplied to the network 9 which embodies a circuit arrangement whereby two out-of-phase components may be derived from a frequency modulated signal. As is well known, such a circuit arrangement may comprise a double-tuned transformer tuned to the center frequency of the frequency modulated signal such as is shown in U. S. A. Letters Patent No. 2,343,- 263, dated March '7, 1944, and issued to J. J. Okrent. 'The output voltages of the auxiliary signals are fed to the electrodes 5 and 6. The phase difference between these output voltages lineally depends upon the momentary frequency of the incoming signal, so that the average anode current also will lineally depend upon this momentary frequency. The high-frequency components in the anode-current of tube I are carried off to earth through the condenser l I, whereas the low-frequency current passes through the loudspeaker l2. In a special construction of this tube the electrodes 5 and 6 have a deflecting effect. In this case it is desirable, as has already been set out, to place an electron lens between these electrodes, thus ensuring that the two controls substantially not act upon each other.

The effect according to the invention is also obtainable by means of tubes having a different construction, if only care be taken that two control electrodes successively block or transmit one and the same electron stream having a definite constant value. As a rule this will only be true if the voltage of the control electrodes does not affect or only slightly affect the cathode current, but only acts upon the distribution of the electron stream among the anode and the other electrodes in the tube. A structurally excellent form of construction is shown in Figs. 5, 6 and 7. In Fig. 5 the reference numeral I9 designates the cathode, of which only one side is furnished with emitting material. From the cathode a stream of electrons, which is formed into a beam, passes to the anode 20. Since the grid 2! has a constant voltage, it is achieved that the electron stream exhibits a saturation characteristic curve. The beam is kept together by means of the rod-shaped electrodes 22 and the particular shape of the two control grids 23 and 24. The first control-grid 23 is surrounded by a screen-grid 25 which consequently shields the grid 23 from the auxiliary electrode 2| and the second control-electrode 24. Between the second control-grid 24 and the anode 20 is provided the screen-electrode 26 and the metal screening 25 acting as a suppressor electrode.

Fig. 6 represents a similar positioning, but here the cathode is surrounded by two grids, the electron stream to one side being used for obtaining the aforesaid effect, whereas the electron stream to the other side is employed for a different purpose, for instance for an amplifying pentodeor triode-system having an anode 28. The electrodes 29 and 30 have such voltages that a saturation characteristic curve is obtained on the one hand, whereas on the other hand a control of the electron stream of the pentode or triode is obtainable.

Both of the electrodes 3| and 32 are controlelectrodes, 33 being a screen-grid which is located both between the electrodes 3| and 32 and between the electrode 32 and anode 34.

In Fig. '7 the formation into a beam of part of the electron stream is obtained by means of screens 35 connected to the cathode, whereas the other part of the electron stream serves for a diode which may be used for automatic gain control or for a separate limiter. The grids 31 and 38 are both control grids. Between the second screen-grid 39 and the anode 49 is placed a suppressor grid 39 whose voltage preferably corresponds to the cathode voltage or has a low positive value. Moreover, materials having a low secondary emission may be used for the gridwires of the other grids.

What I claim is:

1. An electric discharge tube for use in a detector for a frequency-modulated signal from which two out-of-phase components are derived and caused to have a phase-displacement with respect to each other which is a measure of the frequency deviation of the signal, said tube comprising a cathode, an anode, and two control grids disposed in spaced distribution between the oathode and the anode, with a screen grid for each control grid, the screen grids being so spaced that the distance between each control grid and the next screen grid is less than the distance of each control grid from the preceding grid.

2. An electric discharge tube for the use set out and with the construction as claimed in claim 1, in which an additional electrode is provided adjacent the cathode to establish an electron stream from the cathode that shall have a saturation characteristic.

3. An electric discharge tube for the use set out and with the construction as claimed in claim 1, in which each control electrode can transmit or block the electron stream from the cathode according to the potential and polarity of the control electrode relative to the cathode, and an additional electrode adjacent the cathode limits the electron stream to a relatively constant saturation value, whereby the current to the anode is in pulses.

4. An electric discharge tube for the use set out and with the construction as claimed in claim 1, in which an additional electrode is provided adjacent the cathode to establish an electron stream having a saturation characteristic, and a suppressor grid is disposed ahead of the anode and provided with a low voltage positive bias.

5. An electric discharge tube for the use set out and with the construction as claimed in claim 1, in which the tube electrodes comprise, in sequence, the cathode, a suction grid, the first control grid, the first screen grid, the second control grid, the second screen grid, suppressor grid, and anode.

6. A tube as in claim 5, including, additionally, a second anode to receive an electron stream other than that for the first-mentioned anode and independent of the stream to the first-mentioned anode.

ADRIANUS JOHANNE'S WILHELMUS MARIE VAN OVERBEEK.

REFERENCES CITED The following references are of record in the 

